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I have a relatively small molecule in a relatively large system. The SCF converges very badly because of this.

The question is how to increase the speed of convergence. I should not decrease the size of this system (which will definitely help) since I have to exclude electrostatic interactions between the cell.

Thanks.

one of the reasons might be that the box is simply too large. If it is not reasonable to decrease the box size, please also check

--) whether you have an even number of valence electrons. if not, please use ISPIN=2
--) make sure you use the Gamma point only
--) decrease the mixing parameters (incl spin mixing, if you have an unpaired electron)